Scientists from two organizations are looking into the riddle of the ridleys and red tide.

In a paper recently published in the journal Harmful Algae, researchers from Mote Marine Laboratory and the Conservancy of Southwest Florida describe how red tide affects Kemp’s ridley sea turtles, the world’s most endangered sea turtle species.

“This is the first research of its kind,” said study co-author Jeff Schmid, environmental research manager at the Conservancy. “We’re looking at wild, free-swimming turtles. All previous red tide studies have looked at turtles that have washed up immobilized or dead.”

Red tide is a natural phenomenon that occurs when the alga Karenia brevis undergoes a population explosion.

Karenia produces a powerful neurotoxin known as brevetoxin that renders filter-feeders such as oysters, clams and tunicates poisonous and kills fish, marine mammals, birds and sea turtles — brevetoxin killed more than 300 sea turtles in Southwest Florida during red tides in 2005 and 2006.

For the study, Mote and Conservancy researchers took blood from 13 immature Kemp’s ridleys in Pine Island Sound.

Nine turtles were sampled during or immediately after red tides in 2012 and 2013 (the bloom group); four were sampled between the red tides (non-bloom group).

“In Southwest Florida and other places in the Gulf of Mexico, estuaries are where immature sea turtles come to feed and grow up,” Schmid said. “Kemp’s ridleys hatch on beaches in Mexico and south Texas, spend a couple of years on the high seas, move to coastal waters for nine or 10 years, then move out offshore with the adults.”

Scientists weren’t surprised that nonbloom turtles had brevetoxin in their systems because other studies had demonstrated that the toxin remains in the environment long after a red tide bloom.

The question is whether nonlethal levels of brevetoxin can cause chronic physiological problems for the turtles.

“That’s something we’re looking at now,” said Mote’s Justin Perrault, the study’s principal investigator. “We’re looking at brevetoxin in nesting marine turtles. Every female has tested positive for brevetoxin. The brevetoxin is lasting much longer than we thought, and we’re finding evidence of the transfer of brevetoxin from the mother to the egg, so we need to look at the potential effects of red tide on reproduction and live hatchlings.”

“That’s mostly a dietary issue,” Perrault said. “A lot of the turtles we found were feeding on tunicates, which are filter feeders that hold brevetoxin in their tissues, versus dolphins, which feed on fish.”

• The blood of turtles with high concentrations of brevetoxin also contained high levels of a protein called alpha-globulin.

“Alpha globulins are indicative of inflammation,” Perrault said. “So, while the animals look outwardly healthy, the globulins are saying, ‘Yeah, we look fine, but there’s something going on inside.’ ”

• Satellite tags were attached to bloom turtles, and data indicate the animals could detect the presence of Karenia.

“The 2011-2012 bloom remained offshore and alongshore, and the turtles remained in Pine Island Sound,” Schmid said. “Every once in a while, they’d stick their head out a pass and come back in.

“The next red tide was also offshore and alongshore, but it was also within the sound. Turtles moved more within the estuary like they were trying to find an area that didn’t have red tide.”

With extremely high concentrations of Karenia in Pine Island Sound, one tagged turtle moved into the Gulf and took up residence off Sanibel, and another moved into the Gulf off Estero Island.

“The take-home is that they’re avoiding areas with high concentrations of brevetoxin,” Schmid said. “What we don’t have a good handle on is the physiology of how they detect the toxin.”

A lot of work remains to be done on Kemp’s ridleys and red tide in many regards, Schmid said.

“The problem is we can’t plan for red tide because blooms are so unpredictable,” he said.

Kemp’s ridley sea turtle

Scientific name: Lepidochelys kempii

Name: The species was named for Richard Kemp, a Key West fisherman who sent two ridley specimens to Harvard’s Agassiz Museum for identification in 1906. No one knows why the word “ridley” was added to the name of this species and the olive ridley sea turtle (Lepidochelys olivacea).

Status: Endangered

Size: Smallest marine turtle in the world, 100 pounds with a carapace of 24 to 28 inches.

Distribution: In the Gulf of Mexico and Atlantic seaboard from Florida to New England; some reports near the Azores, off Morocco and in the Mediterranean sea.